The phase behavior of mixtures of hydrocarbons, methanol and water is important in gas-condensate reservoirs as well as in gas hydrates. Gas-condensate wells frequently show reduced productivity when operated below the dew point due to the build up of condensate near the well. A combination of high water and condensate saturation results in a significant reduction in the gas relative permeability. Recently, methanol has been used to remove liquid blocking around such wells and restore well productivity. The phase behavior of such fluid mixtures is difficult to model using currently available equations of state.

In this study, phase behavior data were measured both to help interpret coreflood experiments and for testing of equation-of-state models used in compositional simulation studies of methanol well treatments. The Peng-Robinson equation-of-state and the SAFT equation-of-state were used to model this new phase behavior data. The Peng-Robinson equation-of-state does better than the SAFT equation-of-state for hydrocarbon mixtures. The binary interaction coefficients had to be adjusted in both models to match the experimental data when methanol and water are in the mixture. The SAFT EOS required less tuning to match data from hydrocarbon-methanol mixtures. SAFT EOS, as expected, agrees with the data better than the Peng-Robinson EOS when water is in the mixture.

Introduction

Hydrocarbon-water-methanol mixtures are important in diverse applications such as the productivity of gas-condensate wells^1,2 as well as gas hydrate inhibition.³ In gas-condensate reservoirs, a liquid hydrocarbon phase (condensate) drops out when the bottomhole pressure falls below the dew point pressure. This results in a build-up of liquid near production wells and a corresponding decrease in the gas relative permeability and well productivity. Afidick et al.⁴ have reported field data from the giant Arun field in Indonesia that show a reduction in well productivity by a factor of 2 to 4 due to condensate accumulation.

The phase behavior of gas-condensate hydrocarbon mixtures has been extensively studied [Whitson et al.,⁵ Wang et al.,⁶ Sarkar et al.⁷]. However, there have been very few studies [Kokal,⁸ Ng and Robinson⁹] on the influence of water on the phase behavior and properties of gas-condensate fluids. Recently, experimental coreflood studies have shown [Du et al.,¹ Walker²] that methanol treatments can significantly increase the gas relative permeability when condensate and/or water blocking is present. No phase behavior data or modeling under these conditions was available to aid in the interpretation of these experiments. Hydrocarbon-water-methanol mixtures have been studied at lower temperatures where hydrates form.^10,11

In this study, we present the pressure-volume relationships for a synthetic gas-condensate mixture and the effect of methanol and methanol-water mixtures at 145°F. We also model these phase behavior data using a Peng-Robinson equation-of-state and a theoretical equation-of-state, Statistical Associating Fluid Theory (SAFT). The most important property of interest in gas-condensate modeling is the liquid dropout. We also model the effect of the polar components water and methanol on the condensate dropout.